The number of postcode districts with average house prices above the inheritance tax (IHT) threshold has more than doubled in the past five years, Halifax says.

The average house price in 236 (11%) postcode districts in England and Wales is now above the IHT threshold compared to 117 (6%) postcode districts five years ago in 2001.

423 postcode districts have one quarter of sales above the IHT threshold.

Sales above the inheritance tax threshold accounted for at least 25% of property sales in 423 (20%) postcode districts across England and Wales in 2006, compared with 230 (11%) districts in 2001.

They include 54% of London postcode districts and 40% of postcode districts in the South East. All regions of England and Wales have postcode districts with more than 25% of home sales above the IHT threshold.

There are 22 postcode districts that have seen at least a 25 percentage point increase in the proportion of home sales above the IHT threshold. 16 of these districts are outside London and the South East. The largest increase in the proportion of housing sales above the inheritance tax threshold over the past five years has been in NE20 in Newcastle, up from 22% to 61%.

This was followed by PL28 in Padstow with a 39 percentage point increase and EC2Y in the City of London, also with a 39 percentage point rise.

SW1X in Knightsbridge (£1,259,050) and SW7 in Kensington (£1,105,491) are the postcode districts with the highest average house prices above the IHT threshold. All of the top ten most expensive areas are in London, apart from GU25 in Virginia Water (£785,215).

SK9 in Alderley Edge (£376,704) in Cheshire is the postcode district with the highest house prices above the IHT threshold outside the south of England, followed by NE20 near Newcastle upon Tyne (£376,786) and B94 in Solihull (£370,213).

Thirteen postcode districts outside the South of England have an average house price above £300,000, including one in Wales, CF71 in Cowbridge.

There are postcode districts with average house prices above the IHT threshold in all English regions, apart from Yorkshire & the Humber. 36% of postcode districts in London have an average house price above the inheritance tax threshold of £300,000, while 25% of postcode districts in the South East are priced above the IHT threshold.

Since 1995/96 house prices across the UK have increased by 199%, more than double the increase in the IHT threshold, up 95% to £300,000 for 2007/08. If the threshold had increased in line with house price inflation since 1995/96, it would be now be at a level of £460,000, more than 50% above the 2007/08 £300,000 IHT threshold

Over the five years to 2007/08 Halifax calculates that total cumulative inheritance tax (IHT) revenue will be £16.4bn, up more than 50% from the £10.8bn raised by the government in the previous five years.

Revenue raised in 2006 was a record £3.5bn, up £375m (12%) on 2005 when £3.1bn was collected. The government's own projections are for annual IHT revenues to reach £4.1bn in 2007/08.

Tim Crawford, group economist at Halifax, said: "The potential reach of inheritance tax is extending year by year across the country. There has been a doubling in the number of postcodes where the average house price is now above the threshold in the past five years. House prices continue to increase more quickly than the inheritance tax threshold.

"There are 2.3 million properties in the UK valued at more than the inheritance tax threshold. We call on the government to raise the inheritance tax threshold to £460,000. That would account for the significant rise in house prices over the past eleven years."

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